Chemical Reaction on Copper Wheat Cent 1946

Hi, 

I'm wondering what might make a copper cent start showing patches of silver (see pic - sorry about the quality). Or could it be struck on a dime planchet? If so, why would there be copper plating over it? Is it perhaps zinc (although I believe its too early for the zinc copper coated pennies)? I havent unwrapped it since I bought it, and I dont really want to in case I damage it….

Probaby your title is correct.  If another metallic substance had been in contact with the surface, there could have been an electrochemical reaction at the surface.

You could take the coin out and weigh it to see if it's about right for a 1946 cent. That would rule out the other options you mention.